The depletion of natural gas wells is the subject of increasing technical and economic interest. There are several reasons for this growing interest:                It is difficult to predict the time when the natural gas well starts to deplete and to estimate the remaining time until the well is completely exhausted.        Upgrading the facility to an advanced technology is too expensive in relation to the risk connected with the depletion.        Reduced pressure in the gas well requires injection with nitrogen gas and increases the overall liquefaction costs.        
Dr William Cullen, Professor in Chemistry at the Universities of Glasgow and Edinburgh formulated in 1765 his theory of heat and combustion. In 1775 he developed a simple method for producing ice by simply evaporating the air and water vapor from a tank filled with liquid water. Today this refrigeration process is known as evaporation or vacuum cooling.
Evaporation cooling occurs at the liquid-vapor interface. A liquid-to-vapor phase change process requires vaporization heat, which is extracted from the remaining liquid part. Consequently any partial vaporization of a liquid cools the remaining part of the liquid.
Evaporation cooling is applied in gas liquefaction plants, particularly for natural gas liquefaction, to reduce the temperature of the liquefied gas below the condensation temperature. The necessary equipment to introduce evaporation cooling to the LNG liquefaction process is a two-phase LNG expander.
There are numerous references describing the principle of single-phase and two-phase LNG expanders including but not limited to Kikkawa et al., “New Cryogenic Two-Phase Expanders in LNG Production”, March/April 2003; Mukaiboh, Atsushi et al., “Two-Phase Expanders Increase Capacity of LNG Liquefaction Trains”, April 2006; and Chiu, Chen-Hwa et al., “Two-Phase LNG Expanders Replace Two-Phase Joule-Thompson Valves”, April 2004.
FIG. 1 (PRIOR ART) shows a cross section of the design of a two-phase LNG expander such as that manufactured and installed by Ebara International Corporation at the Krio Nitrogen Rejection Plant in Odolanow, Poland “Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions” by Cholast et al. and “Two-Phase LNG Expanders” by Kociemba et al. presented a detailed report on the performance of two-phase LNG expanders at the Krio site in Odolanow/Poland.
There are some important differences in the performance of single-phase and two-phase LNG expanders. Two-phase LNG expanders vaporize a certain amount of LNG to sub-cool the remaining LNG. The reduction of pressure in two-phase expanders is relatively small compared to the pressure difference across a single phase LNG expander, as described in “LNG Expander for Extended Operating Range in Large-Scale Liquefaction Trains” by Kimmel et al. which is hereby incorporated herein by reference in their entirety, without limitations. The performance of single-phase expanders depend only on the mass flow, differential pressure and rotational speed, while the performance of two-phase expanders depends on the composition, temperature, inlet and outlet pressure, volumetric flow and rotational speed. Therefore, changes in the performance characteristic of two-phase expanders have to be adjusted to the momentary process data.
Depleting gas wells are in many cases events which are very difficult to predict in time. Once known, the possible solutions to be applied for depleting gas wells are the same as for new gas wells: To reduce the overall energy consumption for the liquefaction process to a minimum. Each existing equipment of the liquefaction plant has to be analyzed for possible energy savings, and eventually be replaced by more advanced equipment. The costs for upgrades are different for each piece of equipment and some improvements may not be economical for existing plants while other improvements are feasible solutions.
In methods of the prior art, at the location of the natural gas well, wherever it is. The reason for injecting nitrogen into the well is the following: The natural gas at that particular well is not under pressure. To be able to transfer the natural gas out of the well, pressurized nitrogen gas can be injected into the well. Nitrogen is heavier than natural gas and sinks to the bottom of the well. Thus, the lighter natural gas which will be displaced and pushed to the surface by the pressurized nitrogen.
This method is based solely on principles of mechanical engineering and fluid dynamics. The method has the disadvantage to contaminate the natural gas which is a fuel, with nitrogen which is not a fuel, thus decreasing the fuel quality of the natural gas. The expanders described in the literature extract this polluting nitrogen from the LNG by distillation through expansion, a kind of vacuum distillation with nitrogen at its lower boiling temperature. Again, the purpose: is to lift the natural gas out of the well mechanically.
Single-phase and two-phase LNG expanders replacing Joule-Thomson valves increase the LNG production without increasing the energy consumption and are investments that have a payback time of less than six months. In addition, LNG expanders produce electrical energy that reduce the overall energy consumption, to gain the most benefits using LNG expanders.
Non-patent literature TURBO-EXPANDER TECHNOLOGY DEVELOPMENT FOR LNG PLANTS by Chiu does not teach evaporation of nitrogen from a mixture containing LNG in order to cause subcooling of LNG. Rather, Chiu teaches the use of nitrogen as a refrigerant which is compressed and expanded trough several stages of gas expanders to provide necessary conventional refrigeration. Chiu fails to teach or anticipate separation of nitrogen and LNG via evaporation of nitrogen.
Non-patent literature CONTINUOUSLY TRANSIENT OPERATION OF TWO-PHASE LNG EXPANDERS by Finley does not teach evaporation of nitrogen from a mixture with LNG in order to cause subcooling of LNG. Rather, Finley merely references nitrogen rejection plants used for purification of LNG. Finley fails to teach or anticipate subcooling of LNG via evaporation of nitrogen to minimize evaporative cooling or “boil off” losses.